Use correct default color for new border config

Our default theme use default bootstrap variable and is not a real theme like other bootswatch themes. So be sure to use the default value when we do use variables before defaults bootstrap layer applies

Author: cderv

src/resources/formats/html/bootstrap/_bootstrap-variables.scss

@@ -272,9 +272,5 @@ $link-weight: $font-weight-base !default;
 $link-decoration: null !default;
 
 // border colors
-$border-color: mix(
-  if(variable-exists(body-color), $body-color, #fff),
-  $body-contrast-bg,
-  30%
-) !default;
+$border-color: mix($body-contrast-color, $body-contrast-bg, 30%) !default;
 $table-border-color: $border-color !default;